Precision investment casting often involves the construction of a wax pattern assembly that is contained within a ceramic shell mold. The wax pattern assembly is removed from the ceramic shell mold and the resulting shell mold is subsequently filled with molten metal in a further step of the casting process. Removal of the wax pattern assembly from the ceramic shell mold may be effected through the use of heat that causes the wax to melt and thus drain out of the ceramic shell mold. The necessary heat may be obtained through placement of the wax pattern assembly and ceramic shell mold within a high pressure steam autoclave. As an alternate method of imparting heat to the combination, flash firing may be performed. Although capable of heating and therefore removing the wax, such processes may induce stresses into the ceramic shell mold and cause cracking and other defects. The wax pattern assembly has a higher rate of thermal expansion than the ceramic shell mold in which it is located. Heating of these components thus causes greater thermal expansion in the wax than in the ceramic shell mold. Disproportionate thermal expansion of the wax pattern assembly induces a hoop type pressure and stress on the ceramic shell mold thus causing cracks during the dewaxing process which can ultimately lead to metal casting run-outs, metal finning or dimensional scrap.
Precision investment casting parts sometimes include ceramic cores located inside of the wax pattern assembly that often have a complex, nonsymmetrical shape. The thickness of the wax pattern between the ceramic core and the ceramic shell mold is different at different locations. Dewaxing of the wax pattern assembly through the use of an autoclave or by flash firing causes the entire wax pattern surface to heat at the same time. The ceramic core is thus subjected to different pressures at different locations thereon. Pressure differentials on the ceramic core may cause it to shift or break during the dewaxing process. Further, a pressure differential is realized between the portions of the wax pattern assembly near the pour cup and those located farthest from the pour cup. The presence of the pour cup allows pressure to be relieved at those portions of the wax pattern assembly near the pour cup while a greater pressure is imparted to the wax pattern assembly remote from the pour cup. This pressure differential may cause the ceramic core to become dislodged.
In order to reduce defects caused by thermal expansion of the wax pattern assembly, the ceramic shell mold may be made of additional layers so that it is higher in strength and thus resistant to stresses imparted by the thermally expanded wax. However, the use of thicker ceramic shell molds may cause still further casting defects and scrap than if thinner ceramic shell molds were employed. Also, the use of thicker ceramic shell molds may make certain parts difficult or impossible to cast and may increase the cost of the casting process as additional material and time is needed.
Solutions to the aforementioned problems have been proposed in attempting a localized heating of the wax pattern assembly. One such method involves the introduction of a steam and surfactant mixture to a localized area of the wax pattern assembly. A localized temperature elevation is achieved to melt and drain the wax from the ceramic mold. Continued application of the steam and surfactant mixture causes the wax to be melted and drained from the ceramic mold in a progressive manner. The presence of the surfactant causes the liquid wax material to melt partially within the inner surface of the ceramic mold to thus act as a barrier to prevent steam condensate from soaking through the thickness of the ceramic mold and negatively affecting the binder present in the ceramic mold. Although capable of performing a dewaxing process, current methods are time consuming and costly and suffer from other inefficiencies. As such, there remains room for variation and improvement within the art.
Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the invention.